1. Field of the Invention
This invention relates to an electron beam deflection system. The invention has particular, although not exclusive relevance to an electron beam deflection system for a cathode ray tube.
2. Description of the Prior Art
In cathode ray tube displays, the position of the electron beam is usually controlled by a deflection coil or yoke, the current through which creates a magnetic field deflecting the beam. Separate horizontal and vertical deflection arrangements are generally provided. In many cathode ray tube display systems, the display is provided as a raster scan, in which the beam is deflected in a first direction (usually horizontally) from one side of the scanned area of the screen to the other, and then flys back rapidly to commence a second scan in the first direction, but offset in the second direction (usually vertically).
The period provided for the flyback in video signals is generally short. In conventional television signals, for example, the horizontal scan period is 53 microseconds and the period for flyback is 11 microseconds.
The electron beam is generally directed to the centre of the scanning area of the screen. To produce a scan across the screen, a linearly ramping current is passed through the deflection coil, the centre of the ramp being at zero Amps. Since the coil exhibits inductance, the method of producing the ramp in conventional television receivers is to apply a high voltage across the coil, in response to which the current in the coil grows (almost) linearly. At the end of the scan, there is therefore a large current flowing through the coil. To achieve a rapid flyback, it is necessary to reverse the direction of current flow rapidly, to a large value of the opposite sign to that at the end of the scan, so as to bring the beam back to its starting point.
This is conventionally achieved in such television receiver circuits by providing a flyback capacitor which can be switched in circuit with the deflection coil, the capacitance being such that the LC circuit is resonant, with a resonant period of the order of twice the flyback period. At the end of the scan period, the resonant circuit is switched in. The current still flowing through the deflection yoke flows into the flyback capacitor, charging it up. When the inductor is drained and the capacitor is charged, the flyback capacitor starts to discharge through the inductor causing rapidly growing current in the opposite direction. The corresponding voltage across the capacitor during this period therefore rises from a near zero value at the end of the scan period to a high voltage when the inductor current is zero, and then falls back to a near zero level as the capacitor discharges through the deflection coil. After this, the resonant circuit would continue to oscillate, the voltage across the capacitor rising to a high value of opposite sign; to prevent this, a diode is connected across the capacitor to provide a shorting path for current flow in the reverse direction.
The next raster scan across the display area then commences, as before, and at the end of the scan the resonant circuit operates once more to return the beam to the start of the scan.
In other types of display system, for example those used in flight simulators, instead of providing a raster scanned display a calligraphic, vector or "stroke-writing" display is provided in which the beam position is directly controlled from a source of image information. In such applications, the deflection coil is connected to the output of a power amplifier, and the current through the deflection coil is sensed by a sensing resistor to provide a voltage which is fed back and subtracted from the input to the power amplifier, at which a beam controlling voltage signal is applied. The power amplifier thus supplies the current required to reduce the error between the current through the deflection coil and the current required by the corresponding controlling voltage. In fact, such an arrangement can also be used to provide a raster scan if the controlling voltage applied is a sawtooth. In U.S. Pat. Nos. 3,914,654 and 3,499,979, for example, a resonant circuit is provided in conjunction with the power amplifier so that when the power amplifier is operating to provide a raster scan, the flyback can be provided by the resonant circuit. Many modications to this circuit have been published; e.g. U.S. Pat. No. 4,590,408 and EP 0093447 which disclose use of a MOSFET to switch the resonant circuit.
One particular application of a CRT tube is in a telecine scanner. A telecine scanner is used to transfer films (transparencies) onto video tape. This is achieved by scanning each frame of the film with a point source of light, and collecting the light passing through the film in a photomultiplier, the output of which is formatted as a video signal for recording. The scanning point source of light is generated by a cathode ray tube known as a flying spot scanner. In a flying spot scanner, the screen is coated with a phosphor having very little persistence. The cathode ray beam is scanned in a raster pattern across the screen, so that the portion of the screen struck by the travelling beam glows only for a brief time, generating a scanning spot of light. The raster scan operates, typically, at video line and field rates. Modern feature films are designed to be shown on wide cinema screens. The aspect ratio of the film therefore does not match the aspect ratio of domestic television receivers, so that when the film is transferred to video only a part of the film is scanned. The scanned area can be moved by the telecine machine operator to follow the action of the film.
The deflection system for one known telecine machine comprises a horizontal deflection coil driven from a power amplifier, with feedback. The flyback is provided not by using a resonant circuit, but by continuing to operate the amplifier linearly. To modify the position of the raster scanned portion of the phosphor screen, and hence of the film, a DC offset voltage is added at the input to the power amplifier together with the raster generating sawtooth voltage.
Whilst this arrangement is satisfactory, we have realised that a problem will arise when high definition television (HDTV) becomes sufficiently widespread that there is a requirement to be able to transfer films onto tape in the new HDTV format. This will involve scanning at approximately twice the speed of present telecine machines, and will require twice the deflection power to be applied to the deflection coil because the time for flyback is halved. Power amplifiers are expensive, and the power dissipated will also be substantially higher, resulting in unwanted heat and consequential mechanical problems.
Nor is it possible to overcome this problem by using the known resonant flyback circuit of U.S. Pat. No. 3,914,654, for example. The reason for this is that such a circuit can only provide a symmetrical flyback, from one point on the screen to a point an equal distance on the opposite side of the centre of the screen (the centre being the point which corresponds to the position of the undeflected cathode ray beam). Thus, such a flyback circuit cannot deal with an offset scan such as is desirable in telecine machines as discussed above; for a small offset, the linear amplifier might be capable of restoring the beam position at the end of the resonant flyback, but the time taken to do so increases with the magnitude of the offset so that the flyback time as a whole may extend beyond the flyback period available, and beyond a certain level of offset the linear amplifier will be required to provide a greater degree of flyback than if the resonant circuit were not present at all. For this reason, flyback circuits are not considered for applications where an offset to the scan is required.
In U.S. Pat. No. 4,400,652, a magnetic deflection circuit for a CRT display is proposed in which a resonant flyback circuit is provided in combination with a linear feedback amplifier, as in U.S. Pat. No. 3,914,654. The time of firing of a semiconductor switch is controlled to truncate the flyback at some point before the end of the usual half-cycle, if it is desired for the flyback to be interrupted partway. However, there is no disclosure of means for producing a shifted raster scan using this arrangement. It appears that the arrangement would only provide for truncation of the flyback period, and not for an overshoot of the flyback, and hence that it can provide flyback only to a position closer to the centre of the screen than the starting point. Furthermore, since the rate of change of the flyback voltage is very steep, any slight inaccuracies in the timing of the interruption of flyback would result in substantial inaccuracies in the beam position at the end of flyback.